In the commercial production of paper, several measurements are required to insure consistent quality of the final product. One such measurement is the brightness of the pulp. (See TAPPI Standard T217 OS-48.) This measurement is directed to the "whiteness" of the pulp.
Instruments generally available and used for such brightness measurements deal primarily with reflective measurements and are normally off-line instruments. In these off-line instruments, a sample is periodically taken from the pulp washer, dried, and its brightness determined by a reflectance meter. This determination usually takes 20-30 minutes.
One on-line system uses two sensors disposed in the pulp such as on the stock line and measures influx and efflux of a given bleaching stage. The two measurements are compared and the results used to control bleaching chemicals in order to optimize the brightness of the pulp. It appears each sensor measures the back scatter of light in the slurry in the stock line.
A second on-line sensor compares the response of slurry to two wavelengths. Although the exact function of this instrument is not known, it is possible that the two wavelengths are chosen so that one responds to the color of the slurry and one responds to the color of the pulp liquor. This method would be dependent upon the consistency of the pulp. Alternatively, it could be that the response of the liquid is constant for the two wavelengths chosen over the range of process conditions used. In this case, the brightness measurement would be a function of the ratio of the two signals. For a discussion of types and wavelengths generated by reflectance, see Practical Aspects of Diffuse Reflectance Spectroscopy, American Laboratory, March, 1975, pages 55-64.
The meters used for brightness measurements of fibers in a slurry are generally simple reflective measurements of light transmitted into or through the pulp slurry based primarily on back-scatter reflectance.
In measuring the properties of a pulp slurry, the brightness of the pulp fiber is the property to be measured. When simple reflective measurements are used as described above, i.e., back-scatter, such a measurement is dependent upon at least two variables; namely, the fiber color itself which is the property to be measured and the consistency of the pulp slurry. The higher the percent consistency, the greater the back-scatter. A third variable may or may not be important: the pulp liquor color. Thus, present day reflective measurements of on-line instrumentation or of measurements off-line when the measurement is taken from the slurry are affected by the pulp consistency resulting in erroneous readings if the pulp consistency is not specifically analyzed and accounted for.